Raman amplification is a technique well-known in the art for amplifying signal laser light in a fiber optic telecommunications data link. Three approaches to Raman amplification have been investigated to date, defined as “first order”, “second order” and “third order”. In first order Raman amplification, the pump and signal laser radiation are separated by a single Raman stokes shift. The pump and signal laser radiation in second and third order Raman amplification are separated by two and three Stokes shifts, respectively. These latter schemes require the use of “seed” lasers to provide a small amount of radiation in the intervening orders (e.g., in the first order for a second order system, or in the first and second orders for a third order system) in order to bring the pump energy to the optical information signal.
Many Raman amplifier arrangements, particularly second order and third order systems, require relatively high power (e.g., 1 W or more) pump sources. In most cases, conventional semiconductor lasers cannot provide such power, resulting in the use of Raman fiber lasers as pump sources in these higher order systems. One problem with Raman fiber lasers is that they have been found to exhibit relative intensity noise (RIN) levels higher than those found in semiconductor laser diodes. These higher RIN levels are of minimal concern in the counter-pumped configuration where the signal and the pump travel in opposite directions through the fiber amplifier and the signal is therefore amplified by an averaged pump power. However, in a “co-pumped” Raman amplifier where the pump and message signals propagate in the same direction through the fiber amplifier, excessive levels of RIN transfer from the pump to the signal have been problematic. In the co-pumped architecture, pump power fluctuations induce gain fluctuations at the signal laser wavelength, resulting in a transfer of noise from the pump to the signal. This degrades the optical signal-to-noise ratio (OSNR) of the optical information signal and the performance of the data link.
Thus, a need remains in the art for a technique to minimize the problems associated with pump signal RIN in a co-pumped (or counter-pumped) Raman fiber amplifier application.